This invention relates to the surface treatment of metals, defined as degreasing, pickling, phosphatizing or lacquering, in particular to maintaining steady state conditions during such treatment, and especially wherein the surface treatment is conducted in a substantially non-aqueous liquid bath in a tank not hermetically sealed from the ambient atmosphere.
Liquid baths to which this invention is directed are described, e.g., in German patent applications Nos. P 32 09 829.4, P 33 14 974.7 and P 33 24 823.0, incorporated by reference herein, these applications corresponding to U.S. Pat. No. 4,451,301 and U.S. patent application Ser. No. 603,273 filed Apr. 24, 1984.
The liquids on a nonaqueous basis suitable for surface treatment are organic solvents which include, for example, low-boiling halogenated hydrocarbons such as dichloromethane, chloroform, trichlorofluoromethane, dichlorometahne, trichloroethylene, 1,1,1-trichloroethane, 1,1,3-trichloroethane and mixtures of these chlorinated hydrocarons.
Frequently used as solubilizers in pickling and phosphatizing baths are low-boiling alcohols such as methanol, ethanol, isopropanol, propanol, butanol, sec-butanol, tert-butanol, n-pentanol, sec-pentanol, hexanol, heptanol, octanol, 2-ethylhexanol, nonanol, decanol, undecanol, dodecanol and their mixtures.
In addition, such formulations often also contain stabilizers such as quinones, phenols, nitrophenols or nitromethane.
The following compounds are also frequently used as inhibitors: nitroureas, thioureas, methylthiourea, ethylthiourea, dimethylthiourea, diethylthiourea and alkylated thioureas.
Still further, pyridine and picric acid can optionally be used as accelerators.
The main solvent, i.e. the chlorinated hydrocarbon, in pickling and phosphatizing solutions is generally present at 60 to 85% by weight, and the aqueous phosphoric acid at 0.1 to 2% by weight, based on the total solution.
In degreasing baths, the chlorinated hydrocarbons are generally present in a concentration over 95% while in dipping lacquer baths, the solvent portion as a rule is between 30 and 70%, the remainder consisting essentially of bonding agents and pigments.
This invention relates especially to treatment tank facilities in which the baths are based on organic solvent systems containing individual reactive components and from which other components of the baths must be discharged continuously or periodically, as is the case, e.g., pickling or phosphatizing facilities on a solvent basis. For these baths, a virtually constant composition of the treatment bath should be maintained in every operating phase even though, operating continuously in a nonfixed ratio, on the one hand reactive components are consumed and on the other hand, other components are discharged.
Commercial treatment tank facilities for the surface treatment of materials and other materials are known which consist of a tank having a lower heatable zone to receive a treatment bath and an upper cooling zone to produce a cold gas cushion over the optionally boiling bath so as to mitigate solvent loss. Below the cooling zone and/or within the cooling zone there is a condensate channel or, according to a recent proposal, there are also several condensate channels in which the solvent vapors are condensed. Since the cooling zone is not heremetically sealed from the ambient atmosphere, in addition to the solvent vapors,water also condenses from the humidity in the air, so that an actual collected condensate is richer in water than a theoretical condensate corresponding to the state of equilibrium over the liquid phase. To maintain a bath condition as constant as possible, it is therefore conventional to separate the collected condensate optionally, by super cooling, into (1) a phase as predominantly aqueous as possible and (2) a predominantly organic phase. The aqueous phase contains, inter alia, 40 to 90% of water, the remainder consisting essentially of dissolved chlorinated hydrocarbons optionally with the above-mentioned solubilizers and other water-soluble or hydrophilic components from the bath. The organic phase contains 90 to 99% chlorinated hydrocarbons, the remainder consisting essentially of the above-mentioned solubilizers, optionally water and other chlorinated-hydrocarbon-philic components from the treatment bath.
Separation into two phases usually takes place in a phase separation vessel formed as a water separator. While the organic phase can be directly recycled to the bath, the aqueous phase must be discharged to eliminate the excess water. In this connection, the principal problem arises that with the aqueous phase, other components of the bath, are also discharged, such as, e.g., especially the solubilizer and chlorinated hydrocarbon dissolved by the solubilizer. As a result, undesired changes in the composition of the bath occur. If the bath contains water from the start, even more water can be undesirably discharged than is fed in by condensation, so that the bath loses water despite the fact that water is fed in from the ambient atmosphere.
A further change ion the composition of the bath results from the fact that the metal surface to be treated reacts while consuming reactive bath components. Thus, for example, phosphoric acid is consumed in the phosphatizing of metal surfaces; consequently, make-up phosphoric acid must be added to maintain a bath composition as constant as possible. In adding make-up reactive component, e.g., phosphoric acid, a disadvantage arises insofar as the reactive component, e.g., 85% phosphoric acid, cannot simply be proportionately dosed in commercial form into the treatment bath because non-uniform localized concentrations would occur in the bath which in turn would lead to trouble in the surface treatment and to unacceptable inhomogeneities in the resulting phosphate layer.
Dosing of the reactive component, e.g., phosphoric acid, into the treatment bath must therefore take place in a suitable mixture of dilution compatible with the bath. As a result, a principal problem again arises that as a result of the redosing, not only is the consumed phosphoric acid balanced, but also other components, such as, e.g., especially water and solubilizers, optionally also chlorinated hydrocarbons, depending upon choice, are fed into the above-mentioned mixture, which cause shifts in the composition of the bath.
Therefore, considering the aqueous phase, on the one hand various components in addition to water are discharged from the system, and on the other hand, aside from the reactive components, various other components are added in a mixture by dosing to replace the consumed reeactive components. The shifts of the concentration ratios in the composition of the bath connected therewith are not easy to avoid since the rate of flow of the discharged quantities of material is variable and is dependent on the heating and cooling capacity, the thermal convection in the tank, and the tank geometry. Likewise, the dosed quantities of material are variable because they have to conform with the metal surface throughput. Thus, in principle, there is no firm correlation between the resulting discharge quantity and the required dosing quantity because the former is independent of the metal surface throughput and the latter is correlated with the metal surface throughput. The problems connected with keeping the bath composition and the tank volume constant are fundamental in nature and thus far have not been solved.